The present invention relates to a transfer roller for transferring labels of the suction type, as described in the preamble to claim 1.
Known rollers for transferring labels are generally used in reel labelling machines, i.e. machines in which a film of labels is unwound from a reel and routed towards a cutting device, which cuts it to form the labels to be glued onto a container.
The label thus formed always has substantially rectangular or square shape and has two vertical and parallel edges.
The transfer roller, rotating at a predetermined speed about a normally vertical axis, picks up the label and transfers on gluing means and subsequently onto the container to be labelled.
Specifically, the gluing means lay a first strip of glue along a rear face of a first vertical edge of the label and subsequently lay a second strip of glue on a second vertical edge, which will be superposed and glued onto the first edge.
In accordance with a known embodiment variant, existing gluing means are able to lay the glue in points, instead of along one or more strips.
The containers to be labelled are brought in contact with the roller for transferring the label by means of a linear or rotary conveyor, commonly called carrousel.
In accordance with a first prior art technique, the transfer roller is provided with a group of pincers so shaped as to hold the label adhering to the lateral surface of the roller.
Such a transfer roller has the important drawback of being extremely complex to build, since it requires a system of appropriately dimensioned cams to open and shut the pincers during the rotation of the roller.
Secondly, a labelling machine provided with such a roller is not suitable for bottling plants with high productivity, due to the reduced speeds of rotation of the roller.
According to a second prior art technique, within a roller lacking pincers, a plurality of cavities communicating with a lateral surface of the roller through a series of through holes is obtained. Subsequently, through said cavities, a suction of variable and controlled intensity is provided in such a way that the labels can adhere to the lateral surface of the roller during their transfer from the cutting device to the container.
In accordance with a possible known embodiment variant, the lateral surface of the roller is provided with a plurality of recesses to collect the residues of glue which tend to be deposited on the suction holes, in order not to compromise their operation. Said residues are caused by the contact between the label and the gluing means and their consistency is influenced by the speed of rotation of the roller.
Such a transfer roller, known as a suction transfer roller, can be used for labelling both containers made of plastic material, typically PET, and glass containers.
This type of transfer roller has some important drawbacks.
First of all, the intensity and duration of the suction are very complex to control and can negatively influence the cutting precision of the label, especially during the starting transient of the machine.
Secondly, the obstruction of the holes by the label, together with the impossibility to interrupt the suction during the transfer of the label itself from the cutting device to the transfer roller and from the transfer roller to the container, makes difficult the sliding of the label on the lateral surface of the roller. This can compromise the correct positioning of the label on the roller and consequently on the container (especially for labels made of materials with a high friction coefficient) and worsen the operation of the labelling machine, limiting in particular the performance in terms of speed of operation and hourly productivity.
An additional drawback is the difficulty of managing the operating transient which occurs between two consecutive label transferring operations, due to the time required for a predetermined air flow rate to take place within the cavities, after the holes have been freed from the obstruction of the label. In particular, the compressibility of air, together with the intermittence of the suction caused by the presence/absence of the label, generates within the cavity resonance phenomena which compromise the correct positioning and sliding of the label itself on the lateral surface of the transfer roller.